In precure tire retreading operations, a precured tread is applied on a layer of uncured gum rubber to the crown of a buffed and prepared tire casing or carcass. This assembly is held in an autoclave at an elevated pressure and temperature so that the gum rubber layer cures and bonds to the carcass and tread, thus forming a retreaded tire.
A problem in the operation relates to bringing the autoclave to operating pressure during start up. Autoclaves are large containers that require a large volume of compressed air upon starting to reach operating pressure. For example, one commercially available autoclave with a 25 tire capacity has an interior volume of about 905 cubic feet or 6800 gallons. Even after taking into account the volume displaced by the tire assemblies placed in the autoclave, the remaining empty space is quite large. Charging the autoclave with compressed air to operating pressure conditions places a high demand on the compressed air supply at start up of the unit.
In a hybrid retread plant, both precure retread methods, also known as "cold" retreading, and uncured retreads methods, so-called "hot" or mold cure retreading, are used. In mold cure retreading, the replacement tread is not precured, that is, the tread is uncured rubber not formed with the tread pattern. The uncured rubber and carcass assembly are placed in a mold where the tread is molded to the proper form and the tread and gum layer are cured. The mold is typically charged with a heated fluid, such as water, to apply pressure to the assembly. Pressurized air is used to operate the lifting assembly for the mold. The buffers, the equipment that strips the worn tread from the casing, and the builders, the equipment that applies the gum and new tread to the buffed casing, are the same as those used in the precured operation, and consume compressed air for operation. Other equipment includes lifting devices and other devices that help move the tires from station to station in the plant.
A retreading plant typically has a single system to supply compressed air to both the autoclave and all the compressed air-driven equipment in the plant, the buffers and builders and other equipment. Depending on the capacity of the plant, several buffers and builders will be provided. The supply system typically includes a compressor and an accumulator.
At start up, the autoclave intake opens, connecting the large autoclave volume to the compressed air supply. The inlet remains open until the autoclave reaches operating pressure, which, depending on the capacity of the compressed air system, can take several minutes, typically 10 to 15 minutes. The autoclave demand for compressed air is instantaneous, however, and is typically much higher than what the compressor and accumulator can instantly supply. As a result, compressed air is drawn away from the rest of the system, starving the other equipment. The air pressure available to the other equipment therefore falls below the required operating air pressure for the equipment, and the other equipment does not function properly. This results in lost operating time while the autoclave fills, or worse, can result in damage or loss of the tire carcasses being worked on by the equipment because of improper functioning.
Conventional solutions to this problem are costly. A dedicated compressor and accumulator with dedicated supply lines can be provided, but requires both a large expense to purchase the equipment and for operation and maintenance of a separate system. Alternatively, additional compressors and accumulators that activate during autoclave load demand, or larger compressors and accumulators sufficient to meet the usual system demand and the autoclave demand can be provided. These solutions also involve large expenses in purchase, operation and maintenance.
The present invention provides a low-cost, reliable and simple solution that can be applied to existing equipment, without additional compressors or accumulators.
According to the invention, a priority valve system is connected in the compressed air line to monitor and control the flow of compressed air to the autoclave. The inventor realized that the autoclave can fill over an extended period of time without harming performance. The performance of other equipment, to the contrary, is harmed by changes in air pressure. The system prioritizes the compressed air flow to ensure that air at a pressure sufficient to operate the other operating equipment is available during the time the autoclave is filling. If the pressure in the supply line falls below a low pressure limit, air to the autoclave is shut off until the compressor can raise the air pressure above the low pressure limit.
Thus, according to the invention, the supply of compressed air is prioritized to ensure a continuous supply to the equipment other than the autoclave. The autoclave is supplied only when the compressed air supply exceeds the amount need to operate the other equipment. The other equipment remains operable even when the autoclave is being filled, and equipment down time and damage to carcasses are avoided. When no other equipment is being operated, the priority valve can allow a steady flow of compressed air to the autoclave with few, if any, interruptions.
As will be understood by those skilled in the art, the invention can be applied to any compressed air supply system having one or more devices that require immediate, large volumes of compressed air in a way that strains the supply system. In addition, the invention may be used in a system to provide a margin of safety to a critical piece of equipment to protect that equipment from a sudden loss of pressure.
According to a preferred embodiment, the priority valve system includes a solenoid controlled, diaphragm operated valve connected at an inlet to the autoclave. A solenoid controls whether the valve is opened or closed. A switch has a pressure sensor connected to an inlet end of the valve to sense pressure. If the pressure falls below a low pressure limit, the switch activates the solenoid to close the valve, thus shutting off the air supply to the autoclave. With the inlet to the autoclave closed, the compressor is able relatively quickly and easily to raise the pressure in the system above the low pressure limit. The valve remains closed until the sensed pressure rises to a value sufficient to supply both the other equipment and the autoclave.
The switch is a pressure differential switch having a low set point and a high set point. The low set point is set for the low pressure limit below which the air pressure is not sufficient to operate the other equipment. The high set point is established at a point where there is sufficient compressed air to supply both the autoclave and the other equipment.
Another advantage of the invention is that the demand on the compressor is spread out over a longer period of time, which can permit a single compressor to meet the demand. In addition, peak load and operation time for the compressor are reduced, which reduces maintenance requirements.